1. Technical Field
The technical field relates to an acoustic boundary wave device and an electronic apparatus including the same device.
2. Background Art
FIGS. 5 and 6 show sectional schematic diagrams of a conventional acoustic boundary wave device. The device shown in FIG. 5 includes piezoelectric body 2, interdigital transducer (IDT) electrode layer 3, pad electrode layer 4, first dielectric layer 5, and second dielectric layer 6. Piezoelectric body 2 is formed of lithium niobate or lithium tantalate, for example. IDT electrode layer 3 is formed of copper, for example, and placed on piezoelectric body 2. Pad electrode layer 4 is formed on piezoelectric body 2 and is electrically connected to IDT electrode layer 3. First dielectric layer 5 is formed of silicon dioxide, for example, and placed on piezoelectric body 2 so as to cover IDT electrode layer 3. Second dielectric layer 6 is formed of silicon nitride, for example, and placed on piezoelectric body 2 so as to cover first dielectric layer 5. Second dielectric layer 6 is provided with opening 7 which exposes at least a part of the top face of pad electrode layer 4. A thickness of second dielectric layer 6 is greater than wavelength λ of a principal wave excited by IDT electrode layer 3, while a thickness of first dielectric layer 5 is smaller than wavelength λ of the principal wave.
Second dielectric layer 6 covers lateral faces of pad electrode layer 4, and first dielectric layer 5 solidly touches the lateral faces of pad electrode layer 4. The lateral faces of pad electrode layer 4 are thus covered with both of first dielectric layer 5 and second dielectric layer 6 because of the following reason:
In the structure shown in FIG. 6, spaces are provided between pad electrode layer 4 and first dielectric layer 5, and between pad electrode layer 4 and second dielectric layer 6. A metal thin film is formed on the top face of pad electrode layer 4 by sputtering before developing metal plating on the top face. However, the presence of those spaces prevents metal thin film from well attaching to the walls surrounding the spaces. The reliability of connection between pad electrode layer 4 and the metal plating is thus degraded.
In order to overcome the foregoing disadvantage, the lateral faces of pad electrode layer 4 are covered with first dielectric layer 5 and second dielectric layer 6 so that the spaces therebetween can be eliminated, as shown in FIG. 5. This structure allows the metal thin film to well attach to the inner wall of second dielectric layer 6 surrounding opening 7. The connection reliability between the acoustic boundary wave device and an external electronic component (not shown) via the metal plating can be thus improved.
The acoustic boundary wave device shown in FIG. 5 includes, e.g. pad electrode layer 4 of which lateral faces are made of copper, first dielectric layer 5 made of silicon oxide, and second dielectric layer 6 made of silicon nitride. The metal forming the lateral faces of pad electrode layer 4 diffuses into first dielectric layer 5 more readily than into second dielectric layer 6. The metal thus diffuses exhaustively into first dielectric layer 5, so that the resistance of pad electrode layer 4 becomes greater. As a result, electricity passing characteristics of the acoustic boundary wave device is degraded.
The metal diffusion from the top face of IDT electrode layer 3 into first dielectric layer 5 can be prevented with ease by providing the top face of IDT electrode layer 3 with a diffusion preventive layer (not shown). IDT electrode layer 3 is placed lower than pad electrode layer 4, so that the diffusion preventive layer can be readily and positively attached to the lateral faces of IDT electrode layer 3. The metal diffusion from the lateral faces of IDT electrode layer 3 into first dielectric layer 5 can be thus prevented with ease. However, pad electrode layer 4 is placed higher than IDT electrode layer 3 and first dielectric layer 5, so that the diffusion protective layer resists attaching to the lateral faces of pad electrode layer 4. It is thus difficult to prevent the metal forming the lateral faces of pad electrode layer 4 from diffusing into first dielectric layer 5.